The invention relates to directional couplers for diverting or distributing very high frequency signals, the transmission of which requires the use of microstrip lines.
Couplers are known for microstrip lines. Most have a narrow pass band and low directivity, for example 12 dB over 1 or 2 octaves, and/or high losses, this latter case being particularly that of distributors which use resistors. Now, there are applications which require a coupler usable in a very wide frequency band and having a high directivity. Examples of such couplers are those for community antennae and for teledistribution networks, for which it is anticipated that the frequencies will develop towards higher values. In practice, there is a need for a coupler capable of operating in a frequency range extending over more than 5 octaves and having a high directivity.
French 2,276,705 describes a coupler for a strip line formed by a strip line section whose core or conductor is locally close to the core of the main line; such a coupler is not adapted to sufficient directivity in a wide frequency range.
Another prior art directional coupler for a microstrip line (German 2,658,364) comprises two parallel coupled lines extended by elements forming capacitors and replaceable by discrete components. Thus improvement in directivity is obtained but without appreciably increasing the pass band.
According to U.S. Pat. No. 3,416,102 (Hamlin), a coupler for tapping a coaxial cable may have a wire section which, over part of its length, is parallel to the central conductor of the coaxial cable and is in contact therewith. An end portion at least of the take-off section may be at an oblique angle so as to facilitate insertion of the section. The obliqueness of the insertion hole has no other purpose.
There exists no relationship between the latter coupler and those concerned by the invention. Their modes of propagation are entirely different: in one case, there is a coaxial structure which it is desired to modify as little as possible so as to avoid impairing the propagation conditions and the performances, whereas in the other, there is a disymmetric structure (microstrip conductor and mass plane) whose performances are improved. In one case, we have a homogeneous or substantially homogeneous dielectric and in the other case we have a non homogeneous dielectric, formed of two elements (substrate and air).